1. Field of the Invention
The present invention relates to the improvements of an exhaust gas recirculation control system for an internal combustion engine, particularly to techniques for feedback-controlling an exhaust gas recirculation rate depending upon a flow rate of intake air flowing through an air-intake pipe on diesel engines.
2. Description of the Prior Art
In order to minimize atmospheric pollution, specifically nitrogen oxide (NO.sub.x) emissions, from automotive vehicles, there have been proposed and developed various automotive tune-up and exhaust-emission control techniques. An exhaust-gas-recirculation control system, often abbreviated to "EGR system", is used to reduce NO.sub.x emissions from exhaust gases of the internal combustion engine by way of the fall of combustion temperature, caused by recycling of some of the inert exhaust gas back through an intake manifold. One such EGR system has been disclosed in Japanese Patent Provisional Publication No. 60-230555. On one hand, EGR is useful to decrease the formation of NO.sub.x. On the other hand, undesiredly excessive EGR deteriorates combustion, thus dropping engine power output, and also reducing driveability of the vehicle. In recent years, it is desirable to achieve high-precision EGR control in due consideration of various factors, namely (i) engine temperature (during cold engine start, during warming-up period, or in the engine warmed-up state), (ii) the reduced effective opening of an EGR control valve occurring owing to lubricating oil (engine oil) or a product of combustion (carbon deposits) adhered to the EGR control valve, (iii) changes in an intake-air flow rate occurring due to changes in air density, arising from changes in environment from low-land driving to high-land driving under constant engine speed and load, and (iv) the delay in boost pressure (often called "turbo-lag") on turbo-charged engines in a transient state, such as in a transition from normal-straight ahead driving to heavy vehicle acceleration. To avoid the problem of deviation from a desired EGR rate, resulting from changes in the intake-air flow rate occurring due to the drop in air density from low-land driving to high-land driving, Japanese Patent Provisional Publication No. 59-74364 proposes to feedback-control a desired EGR rate on the basis of a detected value of an actual flow rate of intake air flowing through an intake-air pipe of an induction system of a diesel engine.